The present disclosure relates to a hologram recording and reconstruction apparatus.
In recent years, holographic memories serving as recording and reconstruction apparatuses capable of recording and reconstructing data at a high transfer rate have been developed. The holographic memories utilize the thickness direction of a recording medium. When recording data, the holographic memories generate interference fringes between a reference light beam and a signal light beam according to data to be recorded in which information is two-dimensionally arranged as a page. The holographic memories three-dimensionally record the interference fringes therein at one time. When reconstructing the data, the holographic memories emit the reference light beam to the generated hologram so as to obtain a diffracted light beam. Thus, the holographic memories reconstruct the recorded data from the diffracted light beam.
In general, two types of holographic recording and reconstruction method are known: a coaxial method and a two-light beam method. In the coaxial method, to record data, a signal light beam is obtained by modulating a light beam emitted from a laser light source using a spatial light modulator. At the same time, a light beam emitted from that laser light source is obtained as a reference light beam. These signal light beam and the reference light beam are coaxially disposed in part of the light path thereof. Finally, these two light beams pass through the same objective lens and are emitted to a holographic recording medium. In this way, interference fringes between the signal light beam and the reference light beam are recorded on the holographic recording medium. To reconstruct the data, a light beam serving as a reference light beam is emitted from the laser light source onto the holographic recording medium to obtain a light beam (a reconstruction light beam) diffracted by the holographic recording medium in part of the light path in which the diffracted light beam is coaxial with the reference light beam. Subsequently, the recorded data is reconstructed from the reconstruction light beam (refer to, for example, Nikkei Electronics, Jan. 17, 2005, pp. 106-114).
In contrast, in the two-light beam method, to record data, a signal light beam and a reference light beam propagate in different light paths and are independently emitted to a holographic recording medium. To reconstruct the data, a reference light beam and a diffracted light beam (a reconstruction light beam) propagate in different light paths.
In recent years, a photopolymer has been used for a medium of a recording layer of a holographic recording medium since photopolymers can be produced at low cost and have high durability and high light sensitivity (refer to, for example, Japanese Unexamined Patent Application Publication No. 2004-226821). However, when a recording and readout layer is composed of the photopolymer, a dimensional change in the recording and readout layer occurs due to contraction occurring when a monomer is changed to a polymer or due to contraction or expansion caused by a change in temperature during recording. Thus, the angle and spacing of a hologram (a diffraction grating) three-dimensionally formed as an interference fringe pattern tend to change. The change in the shape of the diffraction grating changes the angle and spacing of the hologram when data was recorded compared with the angle and spacing of the hologram when the data is reconstructed. As a result, the angle at which peak diffraction efficiency is obtained varies.
If the angle at which peak diffraction efficiency is obtained varies due to temperatures at a recording time and at a reconstruction time being different, a sufficient quality of recorded data reconstructed from the holographic recording medium cannot be obtained. To address this issue, for example, the above-described page is not reconstructed at one time. The page is divided into a plurality of sections, and each section is reconstructed. In such a case, the transfer rate is decreased and the process of recording is complicated, which is significantly problematic. To address this issue, a technology of changing the wavelength of a laser in accordance with temperature has been proposed (refer to, for example, Mitsuru TOISHI et al., “Temperature tolerance improvement with wavelength tuning laser source in holographic data storage”, Technical digest of Optical Data Storage/International Symposium on Optical Memory, 2005, paper ThE5).
However, in addition to the method in which the wavelength of the laser is changed in accordance with temperature, an angle correction method in which the incident angle of a light beam incident on a holographic recording medium is changed may be employed. In this case, in the two-light beam method (and/or in an apparatus by the two-light beam method), since a reference light beam propagates in a light path different from that of a signal light beam and a diffracted light beam, the incident angle of the reference light beam on the holographic recording medium can be easily changed. In contrast, in the coaxial method, since a reference light beam, a signal light beam, and a diffracted light beam pass through the same objective lens, it is difficult to provide a hologram recording and reconstruction apparatus that compensates for the recording and reconstruction characteristics in accordance with a change in temperature. As a result, the bit error rate (BER) at a reconstruction time of the recorded data on the holographic recording medium increases. That is, the BER deteriorates, and therefore, excellent recording and reconstruction characteristics cannot be maintained.